In the game Super Mario Sunshine, for gamecube, it is possible to get under one of those red cannons.
Go to the amusement park stage, and pick any level that takes place after you battle Monty Mole. (as he takes up the cannon otherwise.) When you spawn, turn right, heading away from the entrance to the park, to the grassy area. At the end of it, there should be a grease covered man menning the cannon. Ground pound him, and after a few tries, you will wind up inside the cannon and the man. It is a very easy glitch to do, though it needs one or two tries.
This is the only time where you can enter a cannon without being ported to another area. As a result, there is nothing special inside the cannon itself. Just a small tube, big enough for you to stand in. However, most players are suprised to find that A. the man inside is hollow (you are really standing inside of him) and B. the man has no legs. (also not a suprise when you think about it, as no one is suposed to see the area down there anyway.) When you have had your fun, you can do an ordinary jump to get out.
Why it worksEdit
I have created a theory about why this works, though I am not sure. Normally, when you ground-pound a friendly npc, your character is pushed to the side of them. In this case, you would be pushed to the side of the man in the cannon, but the interior wall of the cannon prevents you from going in the direction the game intends you to. So, the game compramises and your character is shoved inside the cannon and the man.